Summary

Direct heat treatment of two milk types, skimmed and non-standardised, was performed by instant steam infusion and compared with indirect heating. Infusion conditions were temperatures of 72-120°C combined with holding times of 100-700 ms, and indirect heat conditions were 72°C/15s and 85°C/30 s. The activity of indigenous enzymes; alkaline phosphatase, lactoperoxidase, xanthine oxidase and -glutamyl transpeptidase was evaluated. Mainly the infusion temperature was determinant for inactivation. Whey protein denaturation represented by -lactoglobulin increased significantly with infusion temperature. The non-standardised milk had a higher denaturation rate than skimmed milk. The effect of instant infusion on pH and milk fat globule size in relation to whey protein denaturation and association is discussed.